A. Field of the Invention
This invention relates to improving the performance of ink jet print heads in high frequency usage conditions.
B. Description of Prior Art
Under high-frequency firing conditions, ink jet print heads generally experience higher failure rates. There is also a limiting frequency beyond which the ink jets will not fire. Experiments and theory have shown that many of these failures occur from continued motion of the ink in the ink chamber and the ink meniscus of the orifice after the firing of the print head. When the frequency of firing becomes higher, the meniscus does not have time to settle back to equilibrium before the next firing pulse. The motion is caused by the continued “ringing” associated with resonances in the ink jet. These resonances include the resonance of the piezo electric transducer driving element and fluidic resonances in the ink such as the Helmholtz resonance mode and acoustic modes.
One way to minimize the ringing or resonance of the fluid is to decrease the size of the restrictor to dampen the Helmholtz resonance. However, decreasing the size of the restrictor too much may lead to ink starvation. Starvation failure occurs when the mean orifice pressure becomes more negative and, when combined with a transient negative pressure at firing, overcomes the surface tension strength of the meniscus so that air is drawn into the ink chamber.
One way to avoid starvation failure is to apply air pressure above the ink in the reservoir. However, under positive pressure (hydrostatic head), when the jets are not running, ink starts to weep out of the orifices. The surface tension of the meniscus tends to prevent weeping of the ink and the amount of positive pressure that can be held back depends on the size of the orifices. For the size of the orifices typically used in ink jet heads, the pressure cannot be increased beyond about two inches of water when the jets are not running before the ink starts to weep out of the orifices. When the jets are started and the frequency increased, the hydrostatic head can be increased without ink weeping out onto the outside of the orifice plate. For example, in one experimental laboratory embodiment, pressure was increased up to +20 inches of water when the jets were running at 20 kilohertz. (It is common to refer to the increase or decrease of pressure within the ink jet chamber as being an increase or decrease in inches of water. In many cases the density of the ink or other liquid is close to the density of water so the hydrostatic head is about the same. Other units of pressure commonly used are the pascal and the bar. One inch of water is approximately 249 pascals or 2.49×10−3 bars.) In ink jet print heads, the pressure within the ink jet chamber may be varied by the level of ink or other liquid located within the ink jet reservoir above or below the level of ink or other liquid located within the ink jet chamber—i.e. the hydrostatic “head” of ink. In this embodiment, the use of smaller restrictors and high positive pressure allowed the running of the jets up to the piezoelectric driver resonant frequency, which in this experiment, was 42 kilohertz. Thus, a need exists to allow high frequency operation of the print head without weeping or ink starvation.